1. Field of the Invention
This invention relates to improvements in a process and a system for treating a treatable material such as waste containing a large amount of noxious components such as halogen substances and sulfur, under a heat treatment, in order to prevent generation of virulently poisonous dioxins and make waste gas and the treatable material harmless.
2. Description of the Prior Art
General waste such as urban waste (trash), industrial waste, shredder dust, and plastic waste such as polyvinyl chloride contain a large amount of halogen substances (chlorine, bromine, iodine, fluorine, and astatine), particularly chlorine. Accordingly, when such waste or the like is subjected to heat treatment such as incineration, a large amount of noxious chlorine-containing gas such as hydrogen chloride gas and chlorine gas is generated and becomes a cause for producing virulently poisonous dioxins in residue obtained after the incineration and in flying ash contained in waste gas. Such incineration treatment has been made also on waste (such as waste tire) containing sulfur, in which a large amount of sulfur oxides gas (SOx) are generated. Therefore, treatment for such sulfur oxides gas has been accomplished.
In order to remove the above noxious gases, it has been proposed to spray alkali material (such as lime powder) into an incineration furnace supplied with waste (the treatable material), as disclosed in Japanese Patent Provisional Publication No. 54-93864. In this proposition, the sprayed alkali material reacts with chlorine-containing gas generated under incineration of the waste, thereby to form harmless chloride (such as calcium chloride) thus making waste gas harmless.
It has been also proposed that waste is incinerated upon adding calcium-containing alkali material such as lime (CaCO3) or slaked lime (Ca(OH)2), or that the alkali material is filled in a filter through which waste gas is passed, thus removing noxious chloride gas or sulfur oxides gas from the waste gas. Such propositions are disclosed in Japanese Patent Publication No. 2-10341, Japanese Patent Provisional Publication No. 1-296007, and Japanese Patent Provisional Publication No. 59-12733.
It will be understood that the above conventional techniques are intended to once allow noxious gas to be generated, and thereafter to remove the noxious gas.
Furthermore, it has been known that the treatable material is thermally decomposed or subjected to dry distillation, and thereafter residue formed upon thermal decomposition is reduced in volume under carbonization, ashing or the like. One of such treatments is carried out as follows: The treatable material is thermally decomposed in a single rotatable furnace (rotary kiln). Residue discharged from the rotatable furnace is thereafter incinerated in a stoker. Thermal decomposition gas from the rotatable furnace is burned in a re-burning chamber, generating high temperature gas. The high temperature gas (waste gas) is passed through a boiler or the like and then introduced into a reaction tower into which slaked lime slurry is sprayed so as to react with the waste gas. This process is disclosed in Japanese Patent Provisional Publication No. 5-33916.
Another is carried out as follows: Waste (the treatable material) is heat-treated in a rotary treatment furnace under low temperature dry distillation so as to convert the treatable material into low temperature dry distillation gas and thermally decomposed residue. The residue is burnt in a high temperature burning furnace thereby forming slag in a molten state. Then, this slag is cooled to be solidified in a glass state. The dry distillation gas emitted from the rotary treatment furnace is supplied to a boiler, or discharged upon treatment by a filter or a gas purifying device. Such a process is disclosed in Japanese Patent Publication (Tokuhyohei) No. 8-510789.
According to the above treatment processes under incineration, the alkali material is sprayed into the incineration furnace, and therefore treatment of noxious gas is made near a location where the noxious gas is generated; however, the treatment is made after the noxious gas has been once generated. As a result, although the removal effect for chlorine-containing gas can be expected to some extent, a sufficient removal effect cannot be so obtained as to meet a recent strict regulation for preventing air pollution. Additionally, such treatment processes are accomplished under incineration and therefore high in reaction temperature. As a result, it is difficult to maintain a stable reaction. In case that a large amount of the alkali material is sprayed, the alkali material affects burning itself so that unburned parts are formed. Thus, it is impossible to meet the recent strict regulation for preventing air pollution.
According to the above other treatment processes under the dry distillation, the treatable material is thermally decomposed without being burnt, so that unstable factors such as incineration furnace and the like can be readily removable. However, the treatment process in which alkali material is sprayed into the furnace will provide the same effects as those in the treatment process under incineration.
In case that waste gas contains a large amount of noxious gases (particularly chlorine-containing gas and sulfur oxides gas), corrosion of the furnace and gas duct is remarkable so that there is the fear of lowering durability of the waste treating facility and raising gas leak, thus making maintenance of the facility difficult.
Further, in case that the waste contains sulfur component, when the waste is incinerated upon adding calcium-containing alkali material such as CaO to react with sulfur oxides gas, CaSO4 (calcium sulfate) called gypsum is formed. Gypsum solidifies upon absorbing water content, and therefore post-treatment for gypsum becomes difficult.
As appreciated from the above, according to any of the above-discussed treatment processes, noxious gas is once generated from the treatable material at a former step, and thereafter chlorine-containing gas, sulfur oxides gas and dioxins are removed at a latter step by means of a bag filter or burning treatment. Accordingly, it is difficult to sufficiently remove noxious gases and dioxins.
In order to solve the above-discussed problems, the following treatment process has also been proposed: When the treatable material is heat-treated in a treating furnace, a suitable amount of alkali material (treatment agent) liable to react with chlorine component is mixed with the treatable material so as to fix the chlorine component in residual ash thereby obtaining harmless waste gas. The residual ash is rinsed with water or the like thereby removing the chlorine component. This process is disclosed in Japanese Patent Provisional Publication No. 9-155326.
In this process, the treatment for thermally decomposing the treatable material to generate decomposition gas is accomplished in a single treating furnace. In other words, a series of steps for supplying the treatable material into the single treating furnace through the supply inlet and for discharging carbonized material from the single treating furnace through the discharge outlet are accomplished in the single treating furnace. In the series of steps, the treatable material is stirred and subjected to heat treatment (for example, for 1 hour and at a temperature ranging from 300 to 600xc2x0 C.), thereby successively accomplishing the respective steps of drying, thermally decomposing and volume-reducing (carbonizing) the treatable material.
Now, noxious gas containing halogen substances are generated at a temperature ranging from 200 to 350xc2x0 C. upon thermal decomposition of the treatable material, in which the treatable material and the treatment agent react with each other to form harmless salts. However, there is the possibility that a part of the noxious gas is left in its non-reacted state. Additionally, the treatable material is being stirred, and therefore there is the possibility that the non-reacted noxious gas can be involved in the treatable material. In case that the treatable material is heated at a temperature over 350xc2x0 C. so as to form carbonized material, the noxious gas will be absorbed in the carbonized material.
When the carbonized material, the noxious gas and produced dioxins produced exist simultaneously, the carbonized material unavoidably adsorbs these noxious gas and dioxins. It will be appreciated that it is very difficult to remove the noxious and dioxins which have been once adsorbed. Accordingly, it is difficult to reuse the carbonized material, and therefore it is required to bury the carbonized material in a final disposal place, or otherwise to cause the carbonized material to be subjected to another treatment such as a fusion treatment at a very high temperature.
In view of the above, the present invention is intended to cause the noxious substances generated from the treatable material to contact and react with the treatment agent (alkali material) thereby to form harmless salt(s) when the treatable material is thermally decomposed in a heat treating furnace, thus making waste gas and residue harmless. The residue which has been thus made harmless is reduced in volume, for example, under carbonization in another heat treating furnace, thereby making it possible to reuse the residue.
As result of the inventors"" studies on the experiments, the following facts have been revealed: Noxious gas (particularly, chloride gas and sulfur oxides gas) can be effectively prevented from being rolled in the treatable material (residue) by carrying out a decomposition and reaction step (for thermally decomposing the treatable material to generate noxious gas and for causing the noxious gas to react with the treatment agent) and a volume-reduction step (for reducing the volume of the treatable material) respectively in separate heat treating furnaces, as former and latter steps, so that noxious components cannot be left in the treatable material (residue).
Additionally, according to the inventors"" experiments, the following facts have been recognized: In case that a calcium compound such as calcium carbonate is added as the treatment agent, the noxious component removing effect can be expected to some extent as compared with the effect in case that no treatment agent is added. However, in case that alkali metal (such as sodium or potassium) compound is added as the treatment agent, a plurality of the noxious components can be effectively removed.
In other words, it has been known that when halogen substances (particularly, chlorine-containing gas) and alkali material contact with each other, reaction is made therebetween to form harmless halogen salts such as chloride; however, the halogen removing effect is not sufficient. In this regard, a sufficient noxious component removing effect can be expected by adding alkali metal compound as the treatment agent when the treatable material containing chlorine and/or sulfur is thermally treated, in which the noxious component(s) is removed from decomposition gas generated from the treatable material thereby forming harmless salt(s) such as chloride and/or sulfite) while emitting harmless waste gas. This waste gas can be used as fuel. It is a matter of coarse that the waste gas can be discharged to the atmosphere after such a treatment as to remove dust. Additionally, residue formed upon the heat treatment of the treatable material is made harmless, in which the salt(s) left in the residue can be dissolved out by being rinsed with a solution such as water. Furthermore, it has been found that, in case that the treatable material contains metal components, metals and the carbonized material forming part of the residue can be recovered to be reused.
The present invention has been envisaged and accomplished on the basis of the above-discussed knowledge.
Therefore, an object of the present invention is to provide improved process and system for treating a treatable material containing noxious component(s), which can effectively overcome drawbacks encountered in similar convention processes.
Another object of the present invention is to provide an improved process and system for treating a treatable material containing noxious component(s), in which residue and waste gas were made harmless, thereby making it possible to reuse the residue.
A further object of the present invention is to provide an improved process and system for treating a treatable material containing noxious component(s), in which noxious substance (gas) generated from the treatable material under thermal decomposition can be effectively prevented from being rolled in and kept in the treatable material, thus making the residue harmless.
A still further object of the present invention is to provide an improved process and system for treating a treatable material containing noxious component(s), in which a step for thermally decomposing the treatable material to generate noxious substance (gas) and for causing the noxious substance to react with a treatment agent (alkali material), and another step for reducing the volume of the treatable material are carried out respectively in separate heat treating furnaces so as to prevent the noxious substance from being rolled in and kept in the treatable material.
An aspect of the present invention resides in a process for treating a treatable material containing a noxious component, comprising the following steps: (1) carrying out a first (decomposition and reaction) step for the treatable material, the first step including (a) mixing a treatment agent with the treatable material to form a mixture, the treatment agent containing alkali material, and (b) heating the mixture in a first furnace to thermally decompose the treatable material to generate a substance containing the noxious component, the substance contacting and reacting with the treatment agent to form a harmless salt; and (2) carrying out a second (volume-reduction) step for the treatable material, the second step including heating the treatable material in a second furnace separate from the first furnace so as to reduce volume of the treatable material.
Another aspect of the present invention resides in a process for treating a treatable material containing a noxious component, comprising the following steps: (1) carrying out a first step for the treatable material, the first step including (a) mixing a treatment agent with the treatable material to form a mixture, the treatment agent containing alkali metal compound, and (b) heating the mixture in a first furnace at a first temperature in a low oxygen atmosphere to thermally decompose the treatable material to generate a substance containing the noxious component, the substance contacting and reacting with the treatment agent to form a harmless salt; and (2) carrying out a second step for the treatable material, the second step including heating the treatable material in a second furnace separate from the first furnace, at a second temperature higher than the first temperature so as to reduce volume of the treatable material.
A further aspect of the present invention resides in a system for treating a treatable material containing a noxious component. The system comprises at least one first heat treating furnace which includes a first cylinder body having first and second end sections. The first and second end sections are formed respectively with a supply inlet and a discharge outlet. The first heat treating furnace further includes a first mixing and conveying device for mixing and conveying the treatable material supplied through the supply inlet toward the discharge outlet, and a first heating device located outside the cylinder body to heat the cylinder body. The first cylinder body has an axis which extends generally horizontally. At least one second heat treating furnace is provided to be located separate from the first heat treating furnace and includes a second cylinder body having first and second end sections. The first and second end sections are formed respectively with a supply inlet and a discharge outlet. The second heat treating furnace includes a second mixing and conveying device for mixing and conveying the treatable material supplied through the supply inlet toward the discharge outlet, and a second heating device located outside the cylinder body to heat the cylinder body. The second cylinder body having an axis which extends generally horizontally. Additionally, a duct is provided for connecting the discharge outlet of the first cylinder body and the supply inlet of the second cylinder body. In this treating system, a first treatment is accomplished in the first cylinder body of the first heat treating furnace in such a manner as to mix a treatment agent containing alkali material with the treatable material to form a mixture, and to heat the mixture so as to thermally decompose the treatable material to generate a substance containing the noxious component, the substance contacting and reacting with the treatment agent to form a harmless salt. The treatable material in the first cylinder body of the first heat treating furnace is transferred through the duct to the second cylinder body of the second heat treating furnace. Additionally, a second treatment is accomplished in the second cylinder body of the second heat treating furnace in such a manner as to heat the transferred treatable material to reduce volume of the treatable material.